In the formation of wafer-level chip scale package (WLCSP) structures, integrated circuit devices such as transistors are first formed at the surface of a semiconductor substrate in a wafer. Interconnect structures are then formed over the integrated circuit devices. Solder balls are mounted on the surface of the wafer. The wafer may be sawed into dies.
The dies may be bonded to printed circuit board (PCB) directly. Conventionally, the dies that were directly bonded to PCB were small dies. Accordingly, the stresses applied the solder balls that bond the dies to the respective PCBs were relatively small. Recently, increasingly larger dies need to be bonded to PCBs. The stresses occurring to the solder balls thus became increasingly greater, and the methods for reducing the stresses are needed. However, underfill could not be used in the WLCSP to protect the solder balls. The reason is that if the underfill is applied, the bonding between the dies and the PCBs is not re-workable, and the dies can no longer be removed from the respective PCB in the situations that the dies are defective. As a result, if the underfill is applied, the defective dies cannot be replaced with known good dies once bonded, and the entire PCBs fail.